Turkish President Tayyip Erdogan has agreed to back NATO's defence plans for Poland and the Baltic states, Lithuania's president said on Wednesday.

"This is a huge achievement not only for our country, but for the whole region, because it ensures security of our citizens," Lithuanian President Gitanas Nauseda said in a statement.

Turkey had previously said it would oppose NATO's plan unless the alliance accepted Turkey's designation of certain groups as terrorists, including the Kurdish YPG militia.